This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for aligning an optical system employed therein.
In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of an original document. Exposure of the photoconductive surface records thereon an electrostatic latent image of the original document. The electrostatic latent image is then rendered visible by depositing toner particles which adhere electrostatically thereto in image configuration. Subsequently, the toner powder image is transferred to a sheet of support material which may be plain paper or a sheet of transparent thermoplastic material, amongst others. The toner powder image is, then, permanently affixed to the support material. This provides a copy of the original document.
Electrophotographic printing machines frequently employ scanning optics. On occasion, the scanning system has demonstrated a tendency for the scan lamps and lens of the optical system to become misaligned relative to each other due to mechanical vibrations, cable stretching and slippage of the locking mechanism. Hereinbefore, this has had no major impact on copy quality due to the wide latitude of the electrophotographic printing system. However, with the advent of multi-color photographic printing, the system latitude is relatively small. Hence, the misalignment of the lens system and scan lamps produces a series of variations in sensitometry and the final output color copy quality.
Multi-color electrophotographic printing is substantially the same as the heretofore discussed process with the following distinctions. Rather than creating a total light image of the original document, the light image is filtered producing a single color light image which is a partial light image of the original document. The foregoing single color light image exposes the charged photoconductive surface recording thereon a single color electrostatic latent image. This single color latent image is developed with toner particles of a color complementary to the single color light image. Thereafter, the single color toner powder image is transferred to the support material. The foregoing process is repeated a plurality of cycles with differently colored light images and the respective complementarily colored toner particles. Each single color toner powder image is transferred to the support material in superimposed registration with the prior toner powder image, thereby forming a multi-layered powder image thereon. This multi-layered toner powder image is then heated so as to permanently affix it to the sheet of support material.
It is evident that in multi-color electrophotographic printing, misalignments in the optical system will be significantly magnified. One factor causing this is that successive toner powder images must be transferred in superimposed registration with one another to the support sheet. The registration of the toner powder images relative to one another is determined by the orientation of the electrostatic latent image, which, in turn, is dependent upon the alignment of the optical system. Hence, color electrophotographic printing systems employing a scanning optical system may have copy quality variations due to the misalignment of the scan lamps and lens systems.
Accordingly, it is a primary object of the present invention to improve the alignment of optical systems employed in electrophotographic printing machines to optimize system sensitometry and the resulting copy quality.